1. Field of the Invention
The present invention relates to a hologram optical element fabricated by exposing a hologram photosensitive material to two coherent light beams, to a method of fabricating such a hologram optical element, and to an image display apparatus employing such a hologram optical element.
2. Description of Related Art
Conventionally, there have been proposed various image display apparatuses employing a hologram optical element (hereinafter also referred to as “HOE”). FIG. 14 is an illustrative diagram showing such an image display apparatus in a form with its optical path straightened. In this image display apparatus, the light from a light source 101 is converted into a parallel beam by an illumination lens 102, is then diffused by a diffuser plate 103, then enters a liquid crystal display device (hereinafter also referred to as “LCD”) 104, and then emerges therefrom as image light. The image light is then diffracted by a HOE 105 so as to be directed to an optical pupil E. Thus, at the position of the optical pupil E, a viewer can view an image displayed on the LCD 104.
In general, a HOE employed in such an image display apparatus is fabricated by exposing a hologram photosensitive material to two coherent light beams. Specifically, a hologram photosensitive material is irradiated with two light beams each from one of two point light sources disposed on opposite sides of the hologram photosensitive material so that the two light beams interfere with each other; in this way, a HOE having interference fringes is fabricated.
Here, for example according to JP-A-2001-264682, at the time of exposure of a hologram photosensitive material for the fabrication of a HOE, one light source is disposed substantially at the position of the optical pupil at the time of reconstruction. With a HOE fabricated in this way, at the time of reconstruction, when the viewer views the image with no deviation in the pupil position within the optical plane thereof, the direction from which the exposure rays are incident on the hologram photosensitive material at the time of exposure coincides with the direction from which the image is viewed at the time of reconstruction. This allows the viewer to view a satisfactory image with no color unevenness within the viewing angle.
On the other hand, for example according to JP-A-2004-61731, at the time of exposure of a hologram photosensitive material for the fabrication of a HOE, one light source is disposed on the side of the optical pupil at the time of reconstruction opposite from the HOE. With a HOE fabricated in this way, at the time of reconstruction, even when the viewer's pupil deviates within the plane of the optical pupil, depending on the viewing direction, the viewing direction may be close to, or even coincident with, the direction from which the exposure rays are incident on the hologram photosensitive material at the time of exposure. This allows reduction of color unevenness resulting from a deviation in the pupil position.
In recent years, the spread of display devices for displaying the high-definition video of digital broadcast has been picking up pace. Such display devices have a screen with an aspect ratio of 16 (width):9 (height). When a display device with a laterally elongate screen like these is applied to an image display apparatus, at the position of the optical pupil, the viewer is allowed a laterally expanded viewing angle, and this makes color unevenness within the viewing angle more likely. Accordingly, in such an image display apparatus, it is desirable to design characteristics with priority given to the viewing angle with a view to reducing color unevenness in the lateral direction within the viewing angle.
By contrast, in a construction, like that of a head-mounted display (hereinafter also referred to as “HMD”), where an image display apparatus is supported in front of the viewer's eye by a supporting means corresponding to the frame of eyeglasses, while the image display apparatus does not easily move in the lateral (horizontal) direction, it easily moves in the longitudinal (vertical) direction. Accordingly, in an HMD so constructed, it is desirable to design characteristics with priority given to the pupil with a view to reducing color unevenness in the longitudinal direction resulting from a deviation in the position of the viewer's pupil.
Furthermore, in a case where an image display apparatus is used by a plurality of people with different pupil distances (the distance between the right and left eyes), the pupil position varies in the lateral (horizontal) direction from one viewer to another. Thus, on the same principle on which a deviation in the pupil position causes color unevenness, different people recognize colors differently. Accordingly, in a case where use by a plurality of people is assumed, it is desirable to design characteristics with priority given to the pupil distance with a view to reducing the difference among a plurality of people in the recognition of colors in the lateral direction, that is, the direction of the pupil distance.
The foregoing takes up merely a few examples of the characteristics to which priority needs to be given in the lateral and longitudinal directions; in practice, depending on how an image display apparatus is constructed and used, it often occurs that different characteristics need to be given priority in the lateral and longitudinal directions. Thus, it is desirable to design an image display apparatus so that it offers different viewing characteristics in different directions. This leads to enhanced usability.